Species pools and the "hump-back" model of plant species diversity: an empirical analysis at a relevant spatial scale

We investigated the relative roles of productivity, the species, pool, and spatial habitat structure, in determining local species richness (alpha div ersity) of plant communities within a, single, well-defined landscape unit, at spatial and ecological scales where the relationship between community productivity and species diversity often assumes a unimodal or "hump-back" form. At high levels of productivity, the decrease-phase of the unimodal mo del of the diversity-productivity relationship is typically explained as th e dynamic outcome of increased competitive exclusion, but it may also be th e passive consequence of a small pool of species possessing attributes nece ssary to competitively survive in high-fertility environments. We conducted statistical analyses of previously collected data to determine whether var iations in local richness, in the herbaceous vegetation of a Slovakian moun tain valley were best explained by habitat productivity itself (which presu mably leads to more intense competition) or by the sizes of the relevant co mmunity species pools. We also used measures of spatial habitat structure t o investigate the extent to which habitat patchiness influenced patterns, o f species diversity. In the study system, both community biomass and size o f the species, pools contributed significantly to local species richness, b ut the positive effect of the species pools was about twice as important as the negative effect of biomass. The combined area of related associations (alliance area), association perimeter, and habitat patch geometry were all closely related to species, pool size.